(1) Field of the Invention
The present invention relates to an optical measurement device, and more particularly, to an optical measurement device for measuring an optical spectrum.
(2) Description of the Related Art
With recent increase in the amount of communicated information, large-capacity, low-cost optical fiber communication systems have been actively developed. To cope with increasing capacity, WDMs (Wavelength Division Multiplexers) for multiplexing a plurality of wavelengths for transmission have been researched and developed, making the degree of multiplexing higher and higher.
Channel (ch) spacing, which is an index of the multiplexing degree, is standardized by ITU-T. In current ordinary WDM systems, signal with a transmission rate of 10 Gbps per channel is generally multiplexed with a spacing of 100 GHz (about 0.8 nm) or 50 GHz (about 0.4 nm).
In such WDM systems, EDFA (Erbium-Doped Fiber Amplifier), which is an optical amplifier using an erbium (Er3+)-doped fiber (EDF) as an amplification medium, is generally used as a repeater, in order to compensate for the line loss caused during the transmission of optical signal over the optical fiber.
With EDFA, optical signal is allowed to propagate with pump light introduced into the EDF so that the optical signal level may be amplified by the stimulated emission then induced. In optical amplifiers utilizing stimulated emission as the principle of amplification like the EDFA, spontaneous emission takes place irrespective of the presence/absence of input optical signal. Thus, in a system using an EDFA, amplified spontaneous emission (ASE) caused in the amplifier constitutes noise, which deteriorates the bit error rate (BER).
Accordingly, at the stage of system design, OSNR (Optical Signal/Noise Ratio), which is expressed as the ratio in level of optical signal (S) to optical noise (N), needs to be evaluated by using an optical spectrum analyzer as an optical measurement device. What is important in high-accuracy measurement is in what manner optical noise is separated from optical signal to obtain accurate levels of the noise and signal.
In the case of measuring a signal spectrum with an optical spectrum analyzer, an optical spectrum that ought to be shown as thin lines is displayed as a thick line if the spectrum analyzer does not have sufficiently high resolving power, with the result that the tails spread and overlap with adjacent channels, making it impossible to distinguish the optical noise and signal from each other (given two spectral lines of wavelengths λ0 and λ0+Δλ0 (or frequencies f0 and f0+Δf0), the resolving power represents the smallest value of Δλ0 (or Δf0) where the two wavelengths (or frequencies) can be distinguished from each other as two discrete spectral lines).
Especially, in 10-Gbps NRZ (Non-Return to Zero)-modulated WDM systems, the tails of adjacent signal spectra overlap with each other from the outset where the ch spacing is 50 GHz, making it more difficult to distinguish optical noise and signal from each other. Accordingly, an optical spectrum analyzer with extremely high resolving power is needed to measure the OSNR with high accuracy.
Currently, optical spectrum analyzers for use in this field generally adopt dispersion spectroscopy using a monochromator (light dispersion unit=diffraction grating). Also, as techniques for improving the resolving power for an optical spectrum, there has been proposed a conventional technique wherein an intersecting slit is arranged at the slit position so that light spot components scattering in the Y-axis direction may be cut off, to thereby enhance the resolving power (see, for example, Unexamined Japanese Utility Model Publication No. H07-8736 (paragraph nos. [0013] to [0027], FIG. 1)).
Conventional dispersion spectroscopy-type optical spectrum analyzers use a method in which measurement light to be measured is dispersed by a diffraction grating and a part thereof is extracted through a slit to monitor its power. Specifically, a measurement method is employed wherein the slit is fixed with its width decreased to the smallest possible value and the diffraction grating for dispersing the measurement light is rotated to vary the wavelength band of light passing through the slit, thereby measuring the light intensities of the respective wavelength bands.
FIG. 21 illustrates the width of a beam waist formed by a lens. Generally, the focal point to which the rays of light are converged by a lens has a finite spot width, of which the theoretical minimum value is W=(4·λ·L)/(π·d).
FIG. 22 shows the arrangement of a spectrum analyzer. In the illustrated arrangement, if d0=5 cm, L0=30 cm and λ=1550 nm, for example, the spot width is 6 μm. To achieve high resolving power, therefore, the slit width at the light receiving section also needs to be equal to a minimum width of 6 μm. However, the above spot width is a theoretical limit value and in actuality has a greater value, taking the precision of the system and lens shapes, the precision of the diffraction grating, etc. into consideration.
Accordingly, to realize high-resolution optical spectrum measurement, a slit with an extremely small width is needed. It is, however, difficult to obtain an extremely small slit width for structural reasons, giving rise to a problem that it is difficult to attain sufficiently high resolving power.
The conventional technique (Unexamined Japanese Utility Model Publication No. H07-8736) also has an identical basic structure in that the spectral resolving power is enhanced by decreasing the slit width, and thus is unable to achieve sufficiently high resolving power.